A transmissionsolenoid is an electromechanical valve that controls the flow of transmission fluid into and through an automatic transmission. In modern transmissions, a transmission solenoid typically comes in a pack installed in a transmission control unit, transmission control module, or transmission valve body. Transmission solenoids are powered by a voltage or current the supply of which is controlled by the transmission controller or computer. Based on instructions received from the transmission computer, individual transmission solenoids in the pack direct transmission fluid to particular clutch packs or servo valves to control gear shifting in accordance with driving conditions and engine requirements.

In automobiles, solenoid valves are also used in starting systems, security systems, and air conditioning controls, as well as gas cap locking. A solenoid valve, such as a transmission solenoid, generally has nine parts: the valve body, an inlet port, an outlet port, the solenoid coil body, the coil winding, lead wires, a plunger or piston, a spring, and an orifice. The solenoid body is typically cylindrical in shape, made of steel with a metallic finish, and is hollow inside. The solenoid coil is enameled wire wound around a conductive, ferromagnetic material such as steel or iron. The coil and core take the form of a hollow cylinder in the interior of which resides a plunger.

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The orifice is the connection point between the inlet and outlet ports through which the transmission fluid flows. Acting against the force of the spring, the plunger controls the opening and closing of the orifice. The plunger's movement is controlled by a magnetic field and the amount of current flowing through the solenoid valve.

A magnetic field is induced when an electric current flows through wires and contacts in the solenoid coil body into the solenoid valve. Determined by the amount of current flowing into the valve, this magnetic field strengthens or lessens, thus exerting greater or lesser force on the plunger and counteracting spring. This in turn opens or closes the orifice to a greater or lesser degree, thereby increasing or decreasing the amount, and pressure, of fluid flowing through the orifice.

Sensors monitor the fluid pressure in the outlet port and regulate the amount of current flowing into the transmission solenoid valve. If the sensor senses that greater pressure is needed, more current is allowed to flow through the solenoid valve, thus increasing the force of the magnetic field. The plunger then moves upwards against the force of the spring, further opening the orifice, which allows more fluid to flow.

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